The Republican Party has a favorable rating of 31% (unfavorable: 62%). While the two survey questions can't be directly compared, it is instructive that people view current marijuana prohibition more "unfavorable" than the GOP:
CBS News/New York Times Poll. Jan. 11-15, 2009. N=1,112 adults nationwide
"Do you think that the use of marijuana should be made legal or not?"
Should not 52%, Should 41%, Unsure 7%